Liquefaction of gases whose critical temperature is lower than the ambient temperature, is an important field of application of cryogenics. Large gas liquefaction plants are built for LNG production, for space-vehicle launching sites and for other industrial and scientific purposes. Because of this, developing novel methods and devices for gas liquefaction having improved techno-economic characteristics, is a task of great importance. Conventional systems of liquefaction of gases, whose critical temperature is lower than the ambient temperature, produce a mixture of saturated liquid and saturated vapour (see, U.S. Pat. Nos. 4,012,212; 4,147,525; 4,195,979; 4,229,195; 4,456,459; 4,606,744; 4,894,076; 5,473,900, and a book of R. F. Baron, "Cryogenic Systems", Oxford University Press, 1985). This mixture enters a separator where liquid is separated, and vapour--flash gas--returns to a compressor through a heat exchanger utilizing low temperature of vapour for cooling compressed gas to be liquefied. Typically, the mass of the flash gas is 3-10 times more than mass of the liquid. Therefore, reheating and recompressing flash gas requires a large heat transfer surface of the heat exchanger and high supplementary capacity of the compressor. These factors increase capital cost and power consumption of the liquefying systems.
In cases where it is necessary to produce subcooled liquefied gas having a temperature lower than the saturation temperature, conventional liquefying systems include additional elements, for instance additional heat exchanger where boiling at lower pressure liquefied gas extracts heat from the liquefied gas to be subcooled, additional compressor or a vacuum pump, etc. (see, U.S. Pat. No. 4,575,386).
It is an object of this invention to provide a new Straightforward Method and Once-Through-Apparatus for Gas Liquefaction producing subcooled liquefied gas with an easily controlled temperature and without generating any flash gas.